1. Field of the Invention
The present invention relates to a positioning mechanism for stopping and holding a movable part in place, the movable part being moved from one location to another by an electrically, pneumatically, hydraulically or magnetically actuated mover.
2. Description of the Related Art
Conventional industrial machines and particularly producing installations frequently utilize a sizing feed mechanism which can move a movable part such as a workpiece or a movable machine section from one location to a desired location and then stop and hold the moved part or machine section at the desired location. In general, such a sizing feed mechanism comprises a drive mechanism for moving the movable part and a positioning mechanism for stopping and holding the moved part at the desired location. The sizing feed mechanism may be powered electrically, magnetically, pneumatically or hydraulically. The positioning mechanism may be in any form of a frictionally or magnetically braking mechanism for stopping and holding the workpiece or machine section or a simple mechanism including a machine section such as an abutment member located at the desired location, the abutment member being engaged by the workpiece or machine section to stop and hold it at the desired location.
One such frictionally or magnetically braking mechanism is a table feed mechanism in a machine tool such as a milling machine. The table feed mechanism uses an electric motor to rotate a feed screw connected to the table. When the feed screw is rotated by a predetermined amount, a clutch-brake system is actuated to break the transmission of rotation and at the same time to hold the feed screw at the stopped location to prevent the inertial rotation of the feed screw. Thus, the table may be positioned in place. There is also known an automatic feed mechanism adapted to rotate a feed screw stepwise by means of a pulse motor and to hold the feed screw at the desired location under the influence of a magnetic field produced by the pulse motor after the feed screw has been rotated by a predetermined amount. This can also prevent the inertial rotation of the feed screw. Such frictionally and magnetically positioning mechanisms or sizing feed mechanisms can be used only in specific applications wherein the amount of feed is variable and determined with high accuracy.
In other production installations, however, a positioning mechanism including a machine section such as an abutment member arranged to perform the direct mechanical arrestment of the moving part is generally used since it has a simplified and inexpensive structure if used as a sizing feed mechanism. This positioning mechanism can determine the distance of movement in various movers such as the piston rod of a pneumatic or hydraulic cylinder, the feed nut of an electrically actuated rotational feed screw, and the plunger of an electromagnetic solenoid. In many applications, the abutment member is adapted to engage the piston end, nut or plunger end. Therefore, the direct positioning mechanism must include an abutment member arranged in the path of movement of the pneumatical cylinder, hydraulic cylinder, feed screw nut or electromagnetic solenoid and a sensor located parallel to the abutment member for detecting the movable part and cutting it off from a power source.
The above-mentioned braking and positioning mechanisms require expensive parts such as clutches and brakes which possess functional limitations even if a high-speed sequence circuit is used to control them. Furthermore, the clutches and brakes take a long time to respond to the sequence control circuit, so that the movable part will move to an undesired location until it is completely stopped. This results in inaccurate positioning and makes the entire mechanism more complicated and expensive. If a pulse motor is used, accurate positioning operation can be accomplished, but it is required to design a drive mechanism having as small an inertial force as possible or to control the speed of the drive mechanism in a complicated manner. Therefore, the sizing feed mechanism is inevitably complicated and expensive.
On the other hand, a direct positioning mechanism utilizing an abutment member for stopping and holding a movable part is less expensive. If a pneumatic or hydraulic cylinder is used in the sizing feed system, however, the mechanical responsiveness in these cylinders is low. Thus, the position of the pneumatic or hydraulic cylinder stopped and held is not constant, and the accuracy is poor even if a sensor for detecting the stoppage of the movable part actuates at high speed. Since the stroke of the pneumatic and hydraulic cylinder is determined at the time of manufacture, the distance of movement cannot be changed during the operation of the device. The use of the feed nut driven by an electrically rotated feed screw is more general, but requires a more complicated and expensive mechanism.